were enslaved by their Rings, which Sauron held himself. People often forget that the One Ring was in rapport with Sauron, it still exerted his will independant of him and it would take some time for another to bend it's power to their own will, so if the WK possesed it and if it was possible for him to try and use it, Sauron's will would immediately be exerted on the WK and unlike others who may attempt an exercise of such powers retaining their own will, the WK's enslavement would immediately prevent any prolonged attempt.